In an environment poor in neutralising antibodies, cell-mediated response could be used to evaluate immune response after using a PRRS vaccine and more or less effectively predict protection against the virus.

The objective of the following study was to evaluate the cell-mediated response generated in gilts vaccinated with a PRRRS vaccine (UNISTRAIN® PRRS, HIPRA) against different PRRSV strains isolated from clinical outbreaks in the field.

EXPERIMENTAL DESIGN:
The study was conducted in six-months-old gilts, negative for PRRSV, coming from an historically disease-free farm. The animals were distributed between two groups: 75% were intramuscularly vaccinated with UNISTRAIN®PRRS (attenuated live PRRS vaccine, European genotype; strain VP-046 BIS) and the remaining 25% were administered 2 ml of intramuscular PBS (control group).

Blood samples were collected from the animals to obtain peripheral blood mononuclear cells (PBMCs) on days 0, 7, 14, 28, 42 and 56 post-vaccination. The samples were sent to CReSA (Centre de Recerca en Sanitat Animal) to evaluate cell-mediated immune response by measuring IFN-γ-SC from the PBMCs (ELISPOT assay) (figure 1).Heterologous cell-mediated response was evaluated using five genotype 1 PRRSV strains (table 1) recovered from clinical outbreaks (DIAGNOS, HIPRA), which represent a wide range of strains that were isolated in different European countries in different years. Table 1. Year and country of isolation of five field strains used to evaluateheterologous cell-mediated immune respons.

RESULTS:
PRRSV-specific IFN-γ-SC were first detected against all the strains 14 days post-vaccination (figure 2). In the strains isolated in Spain and the United Kingdom, the response peak was found on day 14 post-vaccination; in the Spanish strain, the level found on day 14 was maintained until the end of the study (D56). In the strains isolated in Hungary, the Slovak Republic and Italy, the response peak was found on day 28, after which it declined.

DISCUSSION AND CONCLUSIONS:
The genetic and antigen variability of PRRSV is considered to be the most important factor in explaining the lack of cross reaction between strains, as heterologous protection is usually inconsistent and incomplete (Lager et al., 1999; Mengeling et al., 1999). On the other hand, the percentage of similarity in the ORF5, or even the complete sequence of the strains, is not a useful parameter for predicting the extent of protection provided by a vaccine against a given strain (Díaz et al., 2006; Prieto et al, 2008).

Although the immunity generated by PRRS vaccine is not fully known, evaluation of neutralising antibodies and cell-mediated response is important and has to be taken into consideration if we want to know how a PRRS vaccine works. The role of cell-mediated immunity for viral elimination or for protecting against a challenge has been discussed and shown in several studies (Díaz et al., 2012; Zuckerman et al., 2007; Lowe et al., 2005; Martelli et al., 2009).

Therefore, in the absence of neutralising antibody production, which is common after the administration of a single dose of any attenuated commercial PRRS vaccine (Díaz et al., 2006; Kim et al., 2008; Zuckerman et al., 2007), the cell-mediated immunity generated by a PRRS vaccine could play an important role in protection against the challenge.

The immunisation of all breeders, and especially gilts, is a key point in the control of PRRS. The main objective in this phase is to obtain good immunisation of gilts, which is why this study was designed using six-month-old gilts.

In this study, despite the wide range of strains used, not only regarding ORF5 variability (88-98% similarity) but also the year of isolation and origin, the results show that vaccination with UNISTRAIN® PRRS induces significant cell-mediated immune response against a wide range of PRRSV strains.

FINAL CONSIDERATIONSAfter more than 25 years confronting PRRS, control of the disease remains a constant challenge for porcine production. The approach to PRRS must be multi-strategic in order to be successful. One of the key aspects of control of this disease is the adaptation programmes for gilts, and their vaccination with UNISTRAIN® PRRS has been shown to be a useful and effective tool.